Open Data supplied by Natural Environment Research Council (NERC)

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus ) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus , that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus ) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus ) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus .

James Clark Ross JR47 CTD Data Document

Sampling strategy

*NOTE* cruise report specifies 30 CTD profiles, data from 29 casts were supplied to BODC. This is because although the intention was to sample the 'standard' locations along the drake passage transect, the actual stations sampled lie mid-way between each of these points, reducing the number of stations sampled by one.

Instrumentation and data processing by originator

Sea-Bird processing

*NOTE* Salinity was derived during the initial processing. (See CR p.129 for details) and direct correspondence with the originator explains that Potential Temperature was also a derived parameter (from Pressure, Temperature and Conductivity)

BODC post-processing and screening

Reformatting

The data were recieved from Mark Brandon (OU, formerly BAS) in Pstar format.

The data were converted from PStar format into BODC internal format (QXF) to allow the use of in-house visualization tools. In addition to reformatting, the transfer process (TR360) applied the following modifications to the data: derived SigmaTheta.

The following variables were transferred from the source file

pressure (db)

ptemp - diagnostic only (degrees C); dropped after screening, only channel to be dropped.

Temperature (degrees C)

Conductivity (mS/cm)

Salinity (psu)

Potential temperature (degrees C)

Sigma Theta (kg/m^3)

Screening

The Reformatted CTD data were displayed onto a graphical editor (EDSERPLO) for visualization and flagging. The data supplied was only downcast data, which had already been trimmed to exclude data logged as the sensors were stabilizing at the surface, and pauses before the upcast. No anomalous points were flagged during BODC screening.

Banking

Once quality control screening was complete, the CTD downcasts were loaded into BODC's National Oceanographic database under the ORACLE Relational Database Management System.

World Ocean Circulation Experiment (WOCE)

The World Ocean Circulation Experiment (WOCE) was a major international experiment which made measurements and undertook modelling studies of the deep oceans in order to provide a much improved understanding of the role of ocean circulation in changing and ameliorating the Earth's climate.

WOCE had two major goals:

Goal 1. To develop models to predict climate and to collect the data necessary to test them.

Goal 2. To determine the representativeness of the Goal 1 observations and to deduce cost effective means of determining long-term changes in ocean circulation.

Climate Variability and Predictability (CLIVAR)

CLIVAR is an international research programme investigating climate variability and predictability on different time-scales and the response of the climate system to anthropogenic forcing. Climate variability, its extremes and possible future changes, has a strong impact on mankind. CLIVAR seeks to better understand and predict our climate in order to take precautions and to reduce impacts of climate variability and change on our planet. CLIVAR is one of the major components of the World Climate Research Programme (WCRP). It started in 1995 and will have a lifetime of 15 years.

The specific objectives of CLIVAR are:

To describe and understand the physical processes responsible for climate variability and predictability on seasonal, interannual, decadal, and centennial time-scales

To coordinate the collection and analysis of observations and the development and application of models of the coupled climate system, in cooperation with other relevant climate-research and observing programmes

To extend the record of climate variability over the time-scales of interest through the assembly of quality controlled palaeoclimatic and instrumental data sets

To extend the range and accuracy of seasonal to interannual climate prediction through the development of global coupled predictive models

To understand and predict the response of the climate system to increases of radiatively active gases and aerosols and to compare these predictions to the observed climate record in order to detect the anthropogenic modification of the natural climate signal

Fixed Station Information

WOCE Southern Repeat Section 1B is a section across Drake Passage in the South Atlantic Ocean. The nominal end points of the section (to date) are at 52° 55.74' S, 58° 00.00' W (at the south of the Falkland Islands) and 61° 03.05' S, 54° 33.10' W (off Elephant Island at the north end of the Antarctic Peninsula).

The section was first occupied by the R/V Polarstern in 1992 (Gersonde, 1993). The first UK occupation of SR1b followed on RRS Discovery later the same year. The National Oceanography Centre, Southampton (formerly known as Southampton Oceanography Centre), in collaboration with the British Antarctic Survey, have occupied the section most years since 1993 on the RRS James Clark Ross. Additionally, there were three Spanish occupations on R/V Hespérides in February 1995, 1996 and 1998 (Garcia et al., 2002). A Drake Passage summary report for RRS James Clark Ross cruises between 1993 - 2000 has been produced.

A table of cruises which occupied SR1b is presented below with links to the relevant cruise reports (were available).

Fixed Station Information

Station Name

Drake Passage

Category

Offshore area

Latitude

59° 0.00' S

Longitude

62° 0.00' W

Water depth below MSL

Drake Passage

The World Ocean Circulation Experiment (WOCE, 1990-1998) was a major international experiment which made measurements and undertook modelling studies of the deep oceans in order to provide a much improved understanding of the role of ocean circulation in changing and ameliorating the Earth's climate.

The Drake Passage is the narrowest constriction of the Antarctic Circumpolar Current (ACC) - the largest current in the world and connects all three major oceanic basins both horizontally and vertically, thus being a key control in the global overturning circulation.Within the Drake Passage, two repeat hydrographic sections (SR1 and SR1b) were established by WOCE. These were designed to extend measurements collected earlier by the International Southern Ocean Studies (ISOS) programme and have continued beyond the WOCE time-frame.

The original section was SR1 (which also covers part of the A21 one time survey track). Subsequently, the section was shifted to the east (and designated SR1b) in order for it to lie on a satellite ground track as illustrated in the image below.

In addition to the hydrographic measurements, UK research in Drake Passage also includes a network of coastal and deep tide gauges, analysis of satellite altimeter data, and state-of-the-art global numerical modeling.